在Mac OS X上组装和链接gcc生成的程序集

Question

If I have the following C Code

int main()
{
    return 77;
}

I can generate asm code with the -S option on clang to get the following (Intel syntax)

$clang -O0 -mllvm --x86-asm-syntax=intel main.c -S 

the code then is

    .section    __TEXT,__text,regular,pure_instructions
    .globl  _main
    .align  4, 0x90
_main:                                  ## @main
    .cfi_startproc
## BB#0:
    push    RBP
Ltmp2:
    .cfi_def_cfa_offset 16
Ltmp3:
    .cfi_offset rbp, -16
    mov RBP, RSP
Ltmp4:
    .cfi_def_cfa_register rbp
    mov EAX, 77
    mov DWORD PTR [RBP - 4], 0
    pop RBP
    ret
    .cfi_endproc


.subsections_via_symbols

However neither as, gas or nasm will generate an object file to link with ld... does clang or gcc generate actual good "ready to go" asm? gcc's default assembler is gas (which isn't even installed on mac os x...? isn't it the same for clang).

So how do I manually assemble the asm code and then link it?

Answer 1

1. Yes, gcc generates correct assembly code.

2. It's just as .

3. You should be able to assemble it with as and link with ld :

    as -o example.o example.s ld -macosx_version_min 10.8.0 -o example example.o -lSystem 

   or, probably more easily, by using gcc or clang as the frontend:

    cc -o example example.s 

Edit: Complete working example:

$ cat example.c 
int main()
{
    return 77;
}
$ gcc -S example.c -o example.s
$ as -o example.o example.s
$ ld -macosx_version_min 10.8.0 -o example example.o -lSystem
$ ./example 
$ echo $?
77

OK, since you're using clang, you might want to be using its assembler, too. Mixing & matching toolchains usually ends in tears:

$ clang -cc1as -filetype obj -mllvm --x86-asm-syntax=intel -o example.o example.s
$ ld -macosx_version_min 10.8.0 -o example example.o -lSystem

Answer 2

既然你正在使用clang，为什么不用clang而不是gcc编译呢？

clang main.s -mllvm --x86-asm-syntax=intel -o main

Answer 3

尝试使用.s文件调用gcc。